This invention relates to a process for the removal of solids from liquids wherein a solids laden liquid is passed horizontally through a filter bed vertically disposed in a chamber, and wherein the filter bed is regenerated after the filtering material becomes loaded with the solids. This invention also relates to an apparatus for conducting the process.
The general process of passing a solids-laden liquid horizontally through a filter bed is known from U.S. Pat. No. 4,021,339. In the method disclosed therein, the liquid to be purified is passed horizontally through several granular media filter beds disposed in series, and respectively separated from each other by grid-like partitions. The filter media contained in the filter beds are increasingly finer in particle size in the flow direction of the liquid. The regeneration of the filter beds is accomplished by flushing with a washing medium. In order to conduct the regeneration, each filter bed is provided with a feed means at the bottom thereof, and a discharge means at the top for the washing liquid. To regenerate a filter bed, the feed of solids-laden liquid is shut off. A flashing liquid is then introduced under pressure through the feed means into the filter bed from the bottom, and flows through the filter bed in the vertical direction. As a result of this flow the contaminants are entrained in the flushing liquid which is then discharged through the discharge means at the top portion of the filter bed.
However, this process has the recurring disadvantage that with the use of granular filter material, that only a portion of the surface layer acts as a filter because the surface becomes clogged by sedimentation of a large part of the solids. This problem is known as "blinding off" and on account of such clogging, the filtering procedure must be interrupted prematurely because otherwise, the pressure drop becomes too high, and the active filtering layer is diminished very rapidly. Another disadvantage of this type of conventional process is that filtering of the liquid cannot be conducted on a continuous basis so long as even one filter bed requires regeneration. This is due to the fact that because of the varying particle sizes of the filter media in the individual filter beds, filtering must be conducted through all the beds in order for the process to be effective, and the filter beds can never be regenerated, i.e., freed of the deposits, simultaneously. Instead, the beds must be cleaned of these deposits at different points in time, with resulting long downtimes. Thus, because of these disadvantages, the process is rendered uneconomical. Furthermore, the requirement of providing a separate flushing liquid also represents another drawback.
In another prior art process, a filtration media such as sand is supported on a horizontal bed of gravel, and solid-contaminated liquid is forced to flow by gravity, or pressure, vertically downward through the filter bed. Once the bed becomes loaded with the solid contaminant, the influent flow is terminated and the wash liquid is flowed through the filter media in a direction opposite to the influent flow to remove the deposited solid particles from the bed. Generally, backwash water volumes of about 2.5% of the volume of water treated during the filtering cycle are typical, and the backwash velocity must be sufficient to fluidize the sand bed, and cause inter-particle collisions. Furthermore, this process is also subject to the "blinding off" problem described previously, and backwashing must be conducted earlier than desired to prevent this problem.
It is known from U.S. Pat. No. 4,162,216 to employ a bed of discontinuous, flexible polyurethane particles as the filtration media, with the polyurethane being preferably reticulated, having numerous interconnected cells. In this process, the influent liquid is passed downwardly through the bed as in conventional filtration systems, and the solid contaminants in the liquid are retained on, but primarily within the polyurethane particles. This system provides many advantages relative to the rapid sand or multi-media filtration systems, including a higher solids loading capacity as well as a lower pressure drop across the bed. Furthermore, regeneration is less complex reducing the amount of liquid required by over 75%, as compared to conventional granular filtration systems. However, the system of U.S. Pat. No. 4,162,216 fails to fully utilize the benefits provided by employing polyurethane as a filter media because the design of the system is basically the same as that of conventional vertical filtration sand and multi-media filter systems.
In a related area, U.S. Pat. No. 3,752,762 discloses an apparatus for filtering oil by passing oil-contaminated water horizontally through filter beds comprised of pumice stones. When the pumice stones become loaded with oil they are removed from the filter bed to be regenerated by steam scrubbing. When regenerated, the stones are returned to the filter bed for additional filtering to be conducted. However, this system cannot be used to remove solids from a solids-contaminated liquid, and in this regard, is inapplicable to the invention.